Traditional signs have been based upon printed materials, paper, plastic, metal, etc., and are therefore not programmable. Accordingly, they are not easily changed. In an attempt to overcome this problem, electronically programmable and/or controllable signs have been in existence for many years. For example, liquid crystal diode (LCD) displays, cathode ray tube (CRT) displays, and other electrically-addressable displays will display an image in response to applied electric signals or fields. However, such signs typically require a large amount of electricity, since they must provide illumination in order to be visible to a viewer.
Electrical twisting-cylinder and rotary ball displays, such as those described in U.S. Pat. Nos. 4,126,854 and 4,143,103, incorporated herein by reference in their entirety, have been developed to overcome the problems with previous programmable signs. Twisting-cylinder displays, rotary-ball displays and related displays have numerous advantages over conventional displays, such as LCDs and CRTs, since they are suitable for viewing in ambient light, they retain an image indefinitely in the absence of an applied electric field, and they can be made to be very lightweight and/or flexible. For further advantages of such displays, see U.S. Pat. No. 5,389,945, incorporated herein by reference in its entirety. Such displays are referred to herein as “electric paper” displays. An example of such a display is a SmartPaper™ display, from Gyricon Media, Inc.
One method of applying an electric field to electric paper display elements is by using a thin film transistor (TFT) active matrix array. A TFT active matrix array is composed of an array of TFTs. A TFT is a Metal-Oxide-Semiconductor Field Effect Transistor (MOSFET) implemented using thin film technology. A TFT uses thin films, made of either amorphous silicon (a-Si) or polycrystalline silicon (p-Si), and a glass substrate. Current flows between the source and drain of a TFT when a voltage is applied to its gate. Thus, by connecting the gate of a TFT to the power rail of the TFT active matrix array, a TFT is turned on (at a positive voltage). If the gate of a TFT is connected to the ground rail, the TFT is turned off (at zero voltage).
It is advantageous to use TFT active matrix arrays with electric paper displays because a localized electric field used to rotate one or more rotatable elements within an electric paper display may be generated from each TFT. TFT active matrix arrays are used as active switches for each picture element or pixel. A TFT active matrix array is similar to a DRAM array. In other words, each display element is at the intersection of horizontal and vertical lines and may be addressed by enabling both lines. A TFT-based display may typically employ one transistor for each sub pixel of the display. Three sub-pixels—pertaining to red, green and blue electric paper display elements—may be used to make one pixel in a color display. Alternatively, a single sub-pixel may be used to make one pixel in a monochromatic display. A sub-pixel may include multiple rotatable elements.
Numerous circuits for powering electronic devices exist. FIG. 1 illustrates an exemplary prior art circuit for electrically powering an electronic device. The circuit of FIG. 1 includes a voltage source 110 and a switching element 115. The voltage source 110 possesses a positive terminal 111 connected to a positive input terminal 116 of the switching element 115 and a negative terminal 112 connected to a negative input terminal 117 of the switching element 115. The positive terminal 111 of the voltage source 110 is also connected to a power rail of a first device 120. The negative terminal 112 of the voltage source 110 is also connected to a ground rail of the first device 120. The first device 120 of FIG. 1 is a thin-film transistor (TFT) active matrix array. The TFT active matrix array 120 includes a plurality of TFTs.
The switching element 115 selects between its positive input terminal 116 and its negative input terminal 117 to determine the voltage on its output terminal 118. The output terminal 118 of the switching element 115 is connected to a second device 125: The second device 125 shown in FIG. 1 is an electric paper display. An electric paper display 125 includes an array of rotatable elements, such as bichromal balls, and a conductive layer 126. One or more rotatable elements correspond to each TFT in the TFT active matrix array 120. Likewise, one or more TFTs can correspond to each rotatable element.
The output terminal 118 of the switching element 115 is connected to a conductive layer 126 of an electric paper display 125. If the positive input terminal 116 is selected, then the conductive layer 126 is driven to a positive voltage. If the negative input terminal 117 is selected, then the conductive layer 126 is driven to zero volts. For each rotatable element, if a voltage differential between the conductive layer 126 and a TFT associated with the rotatable element is greater than or equal to a threshold voltage, the rotatable element rotates into an orientation displaying a first hemisphere to a viewer of the electric paper display 125. If the voltage differential is reversed, the rotatable element rotates into an orientation displaying a second hemisphere to a viewer. If the voltage differential between the TFT and the conductive layer 126 is less than a threshold voltage or zero, then the rotatable element maintains its present orientation. Thus, by altering the connection of the gates of each TFT in the TFT active matrix array 120 to the power or ground rails and by selecting the voltage level of the conductive layer 126 of the electric paper display 125, via the switching element 115, the rotatable elements are aligned to display known patterns.
One problem with the implementation of the prior art circuit occurs when the operating voltage for components of the first device 120 is less than the operating voltage for components in the second device 125. In this case, if the operating voltage of the second device 125 were used to power the first device 120 as well, the first device 120 could be damaged and experience a vastly decreased lifespan.
What is needed is an electronic circuit that overcomes this disadvantage of prior art circuits by providing proper voltage levels to all devices within a system.